The teachings of the present invention are applicable to substantially any type of piano including spinets, uprights and the like. For purposes of an exemplary showing, the invention will be described in its application to a grand piano.
In the usual piano action, a felt-covered piano hammer is caused to pivot about a center and strike its respective piano string in response to the force of the player's finger applied near the front end of the corresponding piano key. The rear end of the piano key actuates the hammer operating mechanism, of which there are many types well known in the art. The weight of the piano hammers varies over the scale (a standard piano consisting of 88 keys and hammers) normally from about 10 to 12 grams at the bass end of the scale, to about 3 to 4 grams at the treble end of the scale. The mechanical advantage of the action is less than unity and is such that a displacement of the playing surface of the key of about 9 mm will result in maximum key travel, causing the key to bottom against the felt located beneath its front end, and moving the hammer upward through a vertical distance of the order of 45 mm. This gives a mechanical advantage usually in the range of from about 0.15 to about 0.2. As a result, the force required to depress the key will be several times the weight of the hammer itself, plus an amount representing the weight of the other parts and the internal friction of the piano action. The values of these factors are such that, in the absence of key weights or other compensating devices, the touch force or weight required to just depress the keys might typically range from between about 70 to about 100 grams at the bass end of the keyboard and from about 40 to about 45 grams at the treble end.
Experience has shown that a touch force of from about 70 to about 100 grams is greater than is usually preferred by professional pianists and a touch force of from about 40 to 45 grams is somewhat less than is considered optimum. Surveys over the years have shown that most professional pianists prefer a touch force to be in the range of from about 50 to about 60 grams. Some pianists, however, may prefer a heavier or lighter touch than others, a lighter touch reducing the amount of input work required from the pianist, and a heavier touch providing greater control of the nuances of tone produced by the instrument. In some cases a pianist may prefer a heavier touch for practice and a lighter touch for actual concert performance. There may be occasions too when a different touch would be preferred for different musical compositions, but whatever the preferred value of touch, experience has further shown that pianists generally prefer a constant or nearly constant value of touch force for all the keys of the piano. In other words, it is desirable that the same value of key force or weight should cause any of the keys to descend.
In order to equalize the static touch force or weight of the keys, it has been the usual and conventional prior art practice, in the construction of pianos, to insert lead weights, appropriately positioned, in holes bored in the sides of the piano keys. Typically, these weights weigh about 14 grams each and are approximately cylindrical in shape. In the bass keys, two to five such weights may be required in the front half of each key. In the middle of the keyboard, there may be two or three weights per key. The distance of the weights from the fulcrum of the key is determined so that all of the keys start to descend with approximately the same value of weight or force applied to the playing surface thereof. In the extreme treble, it may be necessary to place the lead weights behind the key fulcrum in order to increase the touch force to the desired value. Thus, to achieve accurate weighting it is necessary to determine the number and optimum position of the weights for each key, individually. This is a time-consuming undertaking, subject to various errors, and after the holes are bored in the keys, it is difficult to correct such errors. Furthermore, as the instrument is used, the hammers will become grooved and it becomes necessary to compensate for this wear by filing away some of the outer felt from the hammers. This, of course, destroys the original accuracy of the weighting. In conventional piano technology there is no practical way to compensate for such changes, or for the change in touch weight that may sometimes accompany changes in ambient relative humidity.
It is important to appreciate that the addition of weights to the keys only compensates statically for the differences in hammer weight between bass and treble hammers. That is, the addition of weights compensates to make the touch force constant only for steady-state or very slowly moving keys. When the piano is played loudly or quickly, the inertial components of force required to accelerate the hammers from rest to some relatively large final velocity make the dynamic input force at the playing surface of the keys much higher than the static force. Although the static force required to raise the hammer may be in the range of from about 50 to about 60 grams, the dynamic key force can be of the order of several kilograms. At these key speeds, the result of adding key weights is to add apparent mass to the key, which makes the touch sensation of finger against key, as the piano is played, different for a piano key having a large number of weights from one with no weights or only a small number of weights. The amount of key inertia apparent to the pianist also depends upon the distance of the weights from the fulcrum of the key. In other words, the use of key weights to compensate for differences in hammer weight has the additional effect of changing the mechanical impedance at the playing surface of the keys so weighted, and does so for each key to an extent that is not independently controllable. As a result, the action may "feel" different to the player, even from one key to the next, depending upon the number and position of weights affixed to each key. From the standpoint of the pianist, this is perhaps the most important disadvantage connected with the current practice of equalizing (static) touch force by means of adding weights to the keys. Nonetheless, it is believed that the concept of normalizing key impedance also, as well as touch force, over the entire keyboard and keeping it from key-to-key within whatever limits prove optimal from the standpoint of the pianist, is not normally considered. In fact, the standard weighting practice makes manipulation of key touch weight and of key impedance impossible, one independently of the other.
Prior art workers have made various attempts at adjusting touch force by means other than key weighting. U.S. Pat. No. 1,866,707 teaches means for adjusting touch force of the individual keys as well as means for collective adjustment of all of the keys. In accordance with the teachings of this reference, each key is provided with a leaf spring affixed to its underside and deflectable by means of an adjustment screw. In addition, a slotted tube extends transversely of the keyboard with the free ends of the key leaf springs inserted into the slot of the tube. The tube is rotatable about its axis within prescribed limits to collectively adjust the touch forces of all of the keys simultaneously. The teachings of this reference have a number of drawbacks. Principal among these are: (1) the use of leaf springs in this configuration is very likely to give the keys an un-piano-like "feel" to the player; (2) static touch weight and dynamic mechanical impedance of a key are not independently controllable in the reference arrangement, and; (3) assembly and disassembly are difficult.
U.S. Pat. No. 2,999,411 is directed to a tensioning device whereby a piano key can be tensioned toward its elevated position to increase the amount of finger force required to depress the key, so as to enable a player to obtain the desired key action. According to this reference, the usually solid piano balance rail fulcrum pin is replaced by a hollow pin having a vertical bore opening at its upper end. A tensioning spring is provided for each key. The tensioning spring comprises a coil of at least one full convolution, from opposite ends of which extend a relatively short vertical arm and a relatively long, generally horizontal arm. The short arm is configured to be received in the balance pin of the key with a frictional fit. The long arm terminates at its free end in an upwardly curved terminal adapted to bear upon the upper surface of the piano key halfway between the balance or fulcrum rail and the capstan. Adjustment of the key action is achieved by vertically shifting the short arm within the hollow balance rail pin. When the short arm is shifted upwardly, spring force of the long arm on the piano key is decreased. Similarly, moving the short arm more deeply into the key supporting pin will increase the force of the long arm upon the piano key. The adjustment means of this reference differs markedly from that of the present disclosure, and no provision is made for adjusting all of the keys, or groups of keys, simultaneously.
The present invention is directed to a touch force adjustment means wherein coil springs (or other compliant members) are used instead of key weights to obtain a constant static touch force from key to key. This is accomplished by prestressing the springs to achieve the desired static key-touch force, as will be described hereinafter. When the springs of the present invention are used to replace the key weights, key impedance can be normalized independently over the entire keyboard and kept within optimal limits from key to key. As will be described hereinafter, the springs are so designed and installed that the change in key force as a result of spring contraction during key playing is essentially negligible. This prevents the action from having an un-piano-like "feel" to the player. The touch force adjustment means of the present invention not only enables adjustment of the individual keys, but also incremental and simultaneous adjustment of all of the piano keys or groups of adjacent piano keys. Furthermore, the touch adjusting mechanism taught herein makes it convenient for the action assembly (consisting of the hammer rail, wippen rail, hammers, wippens, action brackets, and associated parts) to be detached from the keys as a unit, as may be necessary during manufacturing or during piano servicing in the field.
It will be understood that the amount of initial stretch or expansion of the coil springs needs to be different for the various notes of the scale because of the differences in weight between hammers. However, because all of the springs are made to have essentially the same spring constant, the variation in touch weight for each key (when groups of keys are adjusted simultaneously) can be kept nearly the same. Thus, if a 5 gram touch change is desired, a given appropriate adjustment will result in a change of 5 grams on all of the keys adjusted, even though each spring may have a stretched length different from all the others. In order to compensate for differences in key length between the black keys and the white keys, the distances from the key fulcrums to the point of attachment of the springs have been selected to give the same ratio in relation to the key length for all keys. This means that the point of attachment of the springs to the keys will be somewhat closer to the fulcrum for the black keys, than for the white keys, since the black keys are shorter.
Key weights may or may not be used in combination with the touch force adjustment means of the present invention. As will be described hereinafter, there are some instances wherein the use of key weights (for purposes other than equalizing the static touch force or weight of the keys) may be desirable.